Thermally expandable microcapsules have been used for various applications as a design-imparting agent or a weight-reducing agent. They have been also used for paint, such as foaming ink and wallpapers, to achieve weight reduction.
As such a thermally expandable microcapsule, widely known is one in which a thermoplastic shell polymer includes a volatile expansion agent that can be gasified at a softening temperature or lower of the shell polymer. Patent Literature 1, for example, discloses a method for producing a thermally expandable microcapsule that includes a volatile expansion agent. That method includes the step of adding an oily mixture and an oil-soluble polymerization catalyst to an aqueous dispersion medium that contains a dispersant, with stirring to cause suspension polymerization. Here, the oily mixture is a mixture of a monomer and a volatile expansion agent, such as a low-boiling aliphatic hydrocarbon.
Although such a thermally expandable microcapsule obtained by this method is thermally expandable by gasification of the volatile expansion agent at a relatively low temperature of about 80 to 130° C., the expanded microcapsule problematically outgasses during heating at a high temperature or heating for a long period of time, so that the expansion ratio is lowered. In addition, due to insufficient thermal resistance and strength of the thermally expandable microcapsule, so-called “deflation” may occur to collapse the microcapsule at a high temperature.
Patent Literature 2 discloses a thermally expandable microcapsule in which a polymer obtainable by polymerization of a carboxyl group-containing monomer with a monomer having a group reactive with a carboxyl group is used as a shell. Patent Literature 2 reports that such a thermally expandable microcapsule has increased three-dimensional crosslinking density, and therefore, it shows strong resistance to shrinkage, and has significantly improved thermal resistance, even if the foamed shell is very thin.
However, formation of rigid three-dimensional crosslinks upon polymerization disturbs expansion during foaming, so that the expansion ratio is still insufficient.